ANNOUNCER: Children flooding into the U.S. without their parents. What can be done to prevent catastrophe?

From ABC News, This Week with George Stephanopoulos begins now.

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MARTHA RADDATZ, HOST: Good morning. I'm Martha Raddatz coming to you from Baghdad where we're tracking breaking news in a fast moving crisis that could draw the U.S. back into war.

President Obama now considering air strikes as an army of jihadists remains just 60 miles north of this capital. And while there will be no American boots on the ground, just last night the air craft carrier USS George H.W. Bush and two other warships were moved to the Persian Gulf.

We have team coverage of every angle, including the possible homeland security threat.

But we begin here in Baghdad where we arrived this morning to find a city bracing for attack, ready to take up arms, the country where nearly 4,500 Americans gave their lives now on the brink of civil war.

RADDATZ: New recruits, mostly majority Shias from the south of the country, pushing back against the extremist Sunni jihadi group threatening to pull Iraq into a sectarian civil war, exactly the nightmare scenario the U.S. ambassador to Iraq told us he was deeply concerned about when we were in Baghdad earlier this year.

ROBERT BEECROFT, U.S. AMBASSADOR TO IRAQ: The violence has a grinding effect that can inflame these tensions that are there and particularly sectarian ones and that risks a larger conflict.

RADDATZ: Emerging from the chaos next door in Syria the extremists call themselves ISIS for the Islamic State of Iraq and Syria where this group hopes to build the new state under strict Islamic rule, their tactics so brutal that even al Qaeda has disavowed them.

And as their fighters under black flags have marched south through Iraq, the city's have fallen one by one from Mosul in the north to Saddam Hussein's hometown of Tikrit where ISIS claimed Friday they executed some 1,700 captured Iraqi forces more than 200 miles in a matter of days.

Their march causing an exodus of refugees throughout the country.

I've traveled to Iraq more than 20 times in the past decade of war, watching as the U.S. made standing up Iraqi security forces its primary mission.

Back in 2005, a confident General David Petraeus explained the goal for training Iraq's military.